Method of mass producing and packaging integrated optical subsystems

ABSTRACT

Mass production of integrated optical subsystems may be realized by providing a bonding material surround each die in an array of first dies on a wafer. A plurality of second dies are then aligned with the dies on the wafer. The bonding material is then treated to bond the aligned dies. The bonded dies are then diced to form a bonded pair of dies containing at least one optical element, thus forming an integrated optical subsystem. The bonding material may be provided over at least part of the optical path of each first die, over an entire surface of the wafer or around the perimeter of each first die. The second dies may be provided on another wafer. Either die may contain active elements, e.g., a laser or a detector. The optical elements may be formed in the die or may be of a different material than that of the die.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §120 andis a CIP to U.S. application Ser. No.: 08/943,274, filed Oct. 3,1997,entitled “Wafer Level Integration of Multiple Optical Elements”, nowU.S. Pat. No. 6,096,155, and to U.S. application Ser. No.: 08/917,865,filed Aug. 27, 1997, entitled “Integrated Beam Shaper and Use Thereof”,now U.S. Pat. No. 6,128,134, both of which are hereby incorporated byreference in their entirety for all purposes. U.S. Ser. No. 08/943,274,now U.S. Pat. No. 6,096,155 is a continuation-in-part of U.S. Ser. No.08/727,837, filed Sep. 27, 1996 now U.S. Pat. No. 5,771,218.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method of mass producing andpackaging integrated optical subsystems.

2. Description of Related Art

As the demand for smaller optical components to be used in a widervariety of applications increases, the ability to efficiently producesuch optical elements also increases. In particular, there is a growingdemand for optical elements integrated with active elements, such aslight sources and/or detectors. In forming such integrated opticalsubsystems at a mass production level, the need for accurate alignmentincreases. Further, such alignment is critical when integrating morethan one optical element or an optical element with an active element.

Integrated optical subsystems include elements, at least one of which isan optical element, stacked together along the z-axis, i.e., thedirection of the light propagation. Thus, light traveling along thez-axis passes through the multiple elements sequentially. These elementsare integrated such that further alignment of the elements withthemselves is not needed, leaving only the integrated optical subsystemto be aligned with a desired system.

Many optical subsystems require multiple optical elements. Such requiredmultiple optical elements include multiple refractive elements, multiplediffractive elements, refractive/diffractive hybrid elements andpolarizers/analyzers. Many of these multiple element systems were formedin the past by bonding individual elements together or bonding themindividually to an alignment structure.

In bulk or macroscopic optics to be mounted in a machined alignmentstructure formed using a mechanical machining tools, the typicalalignment precision that can be achieved is approximately 25-50 microns.To achieve a greater level of 15-25 microns, active alignment isrequired. Active alignment typically involves turning on a light source,e.g., a laser, and sequentially placing each optic down with uncuredultraviolet (UV) adhesive. Then each part is moved, usually with atranslation stage, until the appropriate response from the laser isachieved. Then the part is held in place and the epoxy is cured with UVlight, thereby mounting the element. This is done sequentially for eachelement in the system.

Alignment accuracies of less than 15 microns for individual elements canbe achieved using active alignment, but such accuracies greatly increasethe amount of time spent moving the element. This increase is furthercompounded when more than one optical element is to be aligned. Thus,such alignment accuracy is often impractical even using activealignment.

In many newer applications of optics, as in the optical headconfiguration set forth in U.S. Pat. No. 5,771,218, and in theintegrated beam shaper set forth in U.S. Pat. No. 6,128,134, both ofwhich are incorporated by reference in their entirety for all purposes,there is a need to make optical systems composed of severalmicro-components and in which the tolerances needed are much tighterthan can be achieved with conventional approaches. In addition torequiring tight tolerances, elements of lower cost are also demanded.The alignment tolerance needed may be 1 micron to 5 microns, which isvery expensive to achieve with conventional methods.

To achieve greater alignment tolerances, passive alignment techniqueshave been used as set forth in U.S. Pat. No. 5,683,469 to Feldmanentitled “Microelectronic Module Having Optical and ElectricalInterconnects”. One such passive alignment technique is to place metalpads on the optics and on the laser and place solder between them anduse self-alignment properties to achieve the alignment. When solderreflows, surface tension therein causes the parts to self-align.However, passive alignment has not been employed for wafer-to-waferalignment. In particular, the high density of solder bumps required andthe thickness and mass of the wafer make such alignment impractical.

Another problem in integrating multiple optical elements formed onseparate wafers at a wafer level arises due to the dicing process forforming the individual integrated elements. The dicing process is messydue to the use of a dicing slurry. When single wafers are diced, thesurfaces thereof may be cleaned to remove the dicing slurry. However,when the wafers are bonded together, the slurry enters the gap betweenthe wafers. Removing the slurry from the gap formed between the wafersis quite difficult.

Integrated elements are also sometimes made by injection molding. Withinjection molding, plastic elements can be made having two moldedelements located on opposite sides of a substrate. Multiple plasticelements can be made simultaneously with a multi-cavity injectionmolding tool.

Glass elements are also sometimes made by molding, as in U.S. Pat. No.4,883,528 to Carpenter entitled “Apparatus for Molding Glass OpticalElements”. In this case, just as with plastic injection molding,multiple integrated elements are formed by molding two elements onopposite sides of a substrate. Glass molding however has disadvantagesof being expensive to make tooling and limited in size that can be used.

To make optics inexpensive, replication techniques are typically used.In addition to plastic injection molding and glass molding discussedabove, individual elements may also be embossed. An example of suchembossing may be found in U.S. Pat. No. 5,597,613 to Galarneau entitled“Scale-up Process for Replicating Large Area Diffractive OpticalElements”. Replicated optics have not been used previously together withsolder self-alignment techniques. For each replication method, manyindividual elements are generated as inexpensively as possible.

Such replication processes have not been used on a wafer level withsubsequent dicing. This is primarily due to the stresses imposed on theembossed layer during dicing. When using embossing on a wafer level,unique problems, such as keeping the polymer which has been embossedsufficiently attached to the substrate, e.g., such that the alignment,especially critical on the small scale or when integrating more than oneelement, is not upset.

Further, these replication processes are not compatible with the waferlevel photolithographic processes. In particular, replication processesdo not attain the required alignment accuracies for photolithographicprocessing. Even if embossing was compatible with lithographicprocessing, it would be too expensive to pattern lithographically on oneelement at a time. Further, the chemical processing portion oflithographic processing would attack the embossing material.

Other problems in embossing onto plastic, as is conventionally done, andlithographic processing arise. In particular, the plastic is alsoattacked by the chemicals used in lithographic processing. Plastic alsois too susceptible to warping due to thermal effects, which isdetrimental to the alignment required during lithographic processing.

SUMMARY OF THE INVENTION

Considering the foregoing background, it is an object of the presentinvention to efficiently mass produce integrated optical subsystems.Such efficient production is accomplished by forming at least part ofthe integrated optical subsystem on a wafer level and aligning thesubsystem prior to the dicing of at least one of the wafers.

It is further an object of the present invention to address the problemsarising when attempting to achieve such wafer level production ofintegrated multiple optical elements. These problems include ensuringaccurate alignment, allowing precise dicing of the wafer into individualdies containing constituent elements of the integrated opticalsubsystem, and providing additional features for allowing easyincorporation of the integrated optical subsystem into an overall systemfor a desired application. In accordance with the present invention,integration with electronics is straightforward, since optics,opto-electronics and electronic components are fabricated using the samebasic technology.

These and other objects of the present invention will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description givesspecific examples, while indicating the preferred embodiments of thepresent invention, are given by way of illustration only, since variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention and wherein:

FIG. 1 illustrates a first embodiment for bonding together two wafers;

FIG. 2 illustrates a second embodiment for bonding together two wafers;

FIG. 3a is a perspective view illustrating wafers to be bonded;

FIG. 3b is a top view illustrating an individual die on a wafer to bebonded;

FIG. 4a illustrates a specific example of bonding two substratestogether;

FIG. 4b illustrates another specific example of bonding two substratestogether;

FIG. 4c illustrates another specific example of bonding two substratestogether;

FIG. 4d illustrates a specific example of bonding three substratestogether;

FIG. 5 is a flow chart of the bonding process of the present invention;

FIG. 6a illustrates a surface to be embossed by a master elementcontaining an embossable material in wafer form;

FIG. 6b illustrates a surface having embossable material thereon to beembossed by a master element in wafer form;

FIG. 7 illustrates a wafer on which optical elements have been formed onboth sides; and

FIG. 8 is a cross-sectional view of a substrate having a hybrid elementconsisting of a microlens with a diffractive element integrated directlythereon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

As can be seen in FIG. 1, a first substrate wafer 10 and a secondsubstrate wafer 12 are to be bonded together in order to provide aplurality of integrated optical subsystems. A wafer is typically a disc,typically 4, 6, 8, or 12 inches in diameter and typically having athickness between 400 microns and 6 mm. However, a wafer may be anysubstrate that contains multiple dies to be subsequently diced to formindividual elements. In particular, a wafer may be made of glass,silica, fused quartz, and other inorganic substances, such as Si, GaAs,that have good optical properties for its intended use.

These wafers have an array of respective elements formed thereon oneither one or both surfaces thereof, including at least one opticalelement. The individual optical elements may be either diffractive,refractive or a hybrid thereof. One of the wafers may contain discretedevices such as active elements, such as laser diodes, e.g., verticalcavity surface emitting lasers (VCSELs), detectors, etc., or otherdevices, e.g., optical fibers, mircoelectronic modules, mirrors.Alternatively, one of the wafers may be a semiconductor wafer containingactive elements and/or drive electronics. When the two wafers to bebonded together are of different materials, they should be bonded usingeither a bonding material of high elasticity, (e.g., a high polymeradhesive of greater than 10,000 centipoise), creating a thicker and moredeformable bond, or a very strong bond, e.g., solder. One of theseoptions should be used to help compensate for the thermal expansiondifferential between the materials of the wafers. Dashed lines 8indicate where the dicing is to occur on the wafers to provide theindividual integrated optical subsystems.

A bonding material 14 is placed at strategic locations on eithersubstrate in order to facilitate the attachment thereof. By surroundingthe optical elements which are to form the final integrated die, theadhesive 14 forms a seal between the wafers at these critical junctions.During dicing, the seal prevents dicing slurry from entering between theelements, which would result in contamination thereof Since the elementsremain bonded together, it is nearly impossible to remove any dicingslurry trapped therebetween. The dicing slurry presents even moreproblems when diffractive elements are being bonded, since thestructures of diffractive elements tend to trap the slurry.Alternatively, if using a bonding material which is transparent and hasa different enough refractive index from that of the material in whichthe optics are formed such that the optics function properly, such abonding material may be provided over the entire wafer to form the seal,e.g., including being in at least part of the optical path of thesystem, rather than just around each die.

Preferably, an adhesive or solder can be used as the bonding material14. Solder is preferable in many applications because it is smootherthan adhesives and allows easier movement prior to bonding. Adhesiveshave the advantages of being less expensive for a number ofapplications, they can be bonded with or without heating, they do notsuffer with oxidation, and they can be transparent.

When using a fluid adhesive as the bonding material, the viscosity ofthe fluid adhesive is important. The adhesive cannot be too thin, orelse it beads, providing indeterminate adhesion, allowing the dicingslurry to get in between the elements on the wafers, therebycontaminating the elements. The adhesive cannot be too thick, or therestoring force is too great and sufficient intimate contact between thesubstrates 10 and 12 to be bonded is not achieved. The fluid adhesivepreferably has a viscosity between 1,000 and 10,000 centipoise. Theadhesive preferably provides at least 100 grams of shear strength inorder to maintain the seal through the dicing process. More strength maybe required depending upon the end use of the integrated system.Satisfactory epoxies include Norland 68 and Masterbond UV 15-7. Theseepoxies have refractive indices that are too close to that of glass touse them over the entire glass wafer. However, if the optics are formedin another material having a sufficiently different refractive indexfrom that of the epoxy, such as photoresist, the epoxy may be providedover the entire wafer. Similarly, if the wafer is made of a materialwhich has sufficiently different refractive index from that of the epoxyto insure adequate optical performance of the elements therein which areon a side of the wafer to be bonded, the epoxy may be provided over theentire wafer.

When a fluid adhesive is employed, it must be provided in a controlledmanner, such as ejected from a nozzle controlled in accordance with thedesired coordinates to receive the fluid adhesive. After alignment ofthe wafers, the entire assembly is cured, thereby hardening the fluidadhesive and completing the bonding.

When solder is used, an electroplating or sputtering process may beemployed. For example, a masking material may be put over the substratewherever the substrate is not to have solder. Then the entire wafer isplaced into a bath or sputtering chamber. Then solder is placed over theentire wafer and the masking material is pulled off, leaving solderwhere there was no masking material. Once the wafers are appropriatelyaligned, the solder is then heated up to reflow. The solder is cooledand allowed to re-harden, thereby completing the bond.

When using the bonding material used alone as shown in FIG. 1 is a fluidadhesive, a more viscous adhesive is needed in order to ensure that thebonding material remains where it is deposited. Even using a viscousadhesive, the adhesive still typically spreads over a relatively largearea, resulting in a need for a larger dead space between elements to beintegrated to accommodate this spread without having the adhesiveinterfere with the elements themselves.

It is also difficult to control the height of the adhesive when theadhesive is used alone. This results in the amount of adhesive beingovercompensated and the height of the adhesive, and hence the separationbetween the wafers, often being greater than desired. The difficultycontrolling the height of the adhesive also results in air being trappedwithin the space containing the optical elements. This arises from theuncertainty as to the height and the timing of when a vacuum is pulledon the wafer pair. This air is undesirable, as it may expand uponheating and disrupt the bond of the elements.

Therefore, an advantageous alternative is shown in FIG. 2, in which onlyan individual integrated optical element of the wafer is shown. Standoffs 16 for each element to be integrated are etched or replicated intothe bottom substrate wafer 12 at the same time the array of opticalelements are made for the substrate wafer 12, and typically will be ofthe same material as the substrate wafer. These stand offs 16 preferablyinclude a trench formed between two surfaces in which the adhesive 14 isto be placed. These trenches then provide precise spacing between thesubstrates to be bonded and provide more of a bonding surface to whichthe adhesive 14 can adhere. This increased surface area also reducesbeading problems.

When solder is used as the bonding material 14, solid stand-offs arepreferably used to provide the desired separation between the wafers.The solder is then deposited in a thin, e.g., 4-5 micron, layer on topof the stand-offs. While the solder could be used alone as shown in FIG.1, it is more feasible and economical to use the solder in conjunctionwith stand-offs.

The use of the stand-offs allows a more uniform and predictable heightto be obtained, resulting in less air being trapped between the bondedelements. A vacuum may now be pulled just before or at contact betweenthe bonding material and the other substrate, due to the reduction invariability of the separation.

The substrate not containing the stand-offs may have notches formedthereon to receive the stand-offs 16 therein. These notches can beformed at the same time any optical elements on that surface are formed.In such a configuration, the stand-offs 16 and the corresponding notcheswill serve as alignment features, facilitating alignment of the wafersto one another.

FIG. 3a shows the two substrates 10 and 12 prior to being bonded anddiced. The individual optical elements 19 to be integrated may consistof one or more optical elements. Further, the optical elements on thewafers may be identical, or may differ from one another. Prior tojoining the wafers 10, 12, the bonding material 14 is placed on at leastone of the wafers in the manner described above. Advantageously, bothsubstrates 10 and 12 include fiducial marks 18 somewhere thereon, mostlikely at an outer edge thereof, to ensure alignment of the wafers sothat all the individual elements thereon are aligned simultaneously.Alternatively, the fiducial marks 18 may be used to create mechanicalalignment features 18′ on the wafers 10, 12. One or both of the fiducialmarks 18 and the alignment features 18′ may be used to align the wafers.

FIG. 3b shows a top view of a substrate 12 to be bonded including thelocation of the surrounding bonding material 14 for a particular element19. As can be seen from this top view, the bonding material 14 is tocompletely surround the individual element, indicated at 19.

For either embodiment shown in FIGS. 1 or 2, the bonding materialprovided either directly or using stand-offs completely seals eachelement to be individually utilized. Thus, when dicing a wafer in orderto perform the individual elements, dicing slurry used in the dicingprocess is prevented from contaminating the optical elements. Thus, inaddition to providing a structural component to maintain alignment andrigidity during dicing, the bonding material seal also makes the dicinga much cleaner process for the resultant integrated dies.

A specific example of an integrated optical subsystem is shown in FIG.4a. A refractive 20 is formed on a surface of the first substrate 12. Adiffractive 22 is formed on a surface of the other substrate 10. Adiffractive 28 may also be formed on a bottom surface of eithersubstrate. The stand offs 16 forming the trenches for receiving theadhesive 14 are formed at the same time as a refractive lens.

When the lens 20 on the wafer 12 is directly opposite the other wafer,the vertex of the lens 20 may also be used to provide the appropriatespacing between the substrates 10 and 12. If further spacing isrequired, the stand offs 16 may be made higher to achieve thisappropriate spacing.

In addition to using the fiducial marks 18 shown in FIG. 3a foralignment of the substrates 10, 12, the fiducial marks 18 may also beused to provide metalized pads 24 on opposite sides of the substratesrather than their bonding surfaces in order to facilitate alignment andinsertion of the integrated multiple optical element into its intendedend use. Such metal pads are particularly useful for mating theintegrated optical subsystems with an active or electrical element, suchas in a laser for use in an optical head, a laser pointer, a detector,etc. Further, for blocking light, metal 26 may be placed on the samesurface as the diffractive 22 itself using the fiducial marks 18.

An alternative optical subsystem incorporating discrete devices providedon a mount substrate is illustrated in FIG. 4b. As shown in FIG. 4b, forsome configurations, it is advantageous to dice one of the wafers firstto form individual dies, passively align the individual dies with theother wafer, provide bonding material to seal the elements of theintegrated optical subsystem and then dice the wafer-die pair. In FIG.4b, the integrated optical subsystem includes a side emitting laserdiode 25, including a monitor diode 29, and a mirror 27 for directinglight from the laser diode 25 to a diffractive optical element 22 formedon the wafer 10, which has previously been diced into individual dies11. The discrete devices 25, 27 and 29 are mounted in the substrate 12.Bonding material 14 seals each subsystem. The dashed lines 8 indicatewhere dicing is to occur. While still requiring individual placement ofdies on the wafer, passive alignment is still effectively employed andthe seal formed around the bonded wafer-die pair still prevents dicingslurring from getting between the wafer-die pair. When providedindividual elements on a mount substrate, the mount substrate containsfiducial marks for each subsystem.

FIG. 4c illustrates an embodiment in which a substrate 12 containingrefractive lenses 20 on one side and diffractive lenses 22 on theopposite side is bonded to another substrate 10, which may or may notcontain additional elements, with the space being completely filled witha bonding material 14, e.g., a low index epoxy. In this example, therefractive lenses 20 are created by reflowing photoresist. Then theselenses 20, 22 remain in the photoresist, i.e., the pattern in thephotoresist is not transferred into the substrate 12. The photoresistlenses may be hardened in a conventional manner. Providing the epoxy 14over the entire wafer allows the epoxy 14 to be spun on the wafer 12,which gives a uniform layer thickness. As evident from the dicing lines8, the encapsulated refractive lenses 20 will be protected fromcontamination by the dicing slurry.

FIG. 4d illustrates a specific embodiment in which threesubstrates/wafers are bonded together. In this configuration, a laserdiode I is provided on a substrate 2, which may be bonded to the opticalsubsystem substrates. The beam output by the laser diode 5 iselliptical. A diffractive element 3 on a substrate 4 collimates the beamalong the fast axis. Another substrate 5 contains no optical elementshaving power thereon and serves as a spacer block. Yet another substrate6 has a diffractive element 7 for focusing the beam onto a fiber 9. Thethicknesses of the substrates 4, 5 and 6 are provided such that the beamis circular on the diffractive element 7. Since there are no opticalelements on the bonding interfaces between any of the substrates, thebonding material 14 may be provided over the entire bonding surface ofeach substrate. By making the substrates containing the diffractiveelements out of a material having a higher refractive index than that ofglass, e.g., silicon, the diffractive elements may be made moreefficient. The bonding of the present invention may be used to create,for example, any of the beam shaper configurations set forth in U.S.Pat. No. 6,128,134.

FIG. 5 shows a flow chart of the general process of bonding together twowafers or a wafer-die pair in accordance with the present invention. Instep 30, a substrate wafer is positioned relative to the bondingmaterial to be distributed. In step 32, the bonding material is appliedto the wafer in a pattern to provide sealing around the elements, eitherdirectly, with the stand-offs 16, or covering the entire wafer. In step34, the second substrate wafer or die is aligned with the firstsubstrate wafer. Just before contact is achieved, a partial vacuum ispulled to remove air from between the substrates. This step is notnecessary when bonding a wafer and a die. In step 36, the wafer or thewafer and die are brought into contact. In step 38, the alignment of thetwo wafer or the wafer and die is confirmed. In step 40, the adhesive iscured or the solder is reflowed and then allowed to harden. Once firmlybonded, in step 42, the bonded wafers or wafer with the dies bondedthereto are diced into the individual elements.

The elements to be bonded together are preferably created by directphotolithographic techniques, as set forth, for example, in U.S. Pat.No. 5,161,059 to Swanson, which is hereby incorporated by reference, forthe diffractive optical elements, or in creating the sphericalrefractive elements by melting a photoresist as taught in O. Wada,“Ion-Beam Etching of InP and its Application to the Fabrication of HighRadiance InGAsP/InP Light Emitting Diodes,” General Electric ChemicalSociety, Solid State Science and Technology, Vol. 131, No. 10, October1984, pages 2373-2380, or making refractive elements of any shapeemploying photolithographic techniques used for making diffractiveoptical elements when the masks used therein are gray scale masks suchas high energy beam sensitive (HEBS) or absorptive gray scale masks,disclosed in U.S. Pat. No. 6,071,652, which is hereby incorporated byreference in its entirety.

Alternatively, these photolithographic techniques may be used to make amaster element 48 which in turn may then be used to stamp out thedesired element on a wafer level in a layer of embossable material 50onto a substrate 52 as shown in FIG. 6. The layer 50 is preferably apolymer, while the substrate 52 can be glass. As used herein, the term“glass” is to include glass, silica, fused quartz, other inorganicsubstances, such as Si, GaAs, that have good optical properties and aredurable, as well as plastic, preferably polycarbonate or acrylic. Thepolymer is preferably a UV curable acrylate photopolymer having goodrelease from a master and good adherence to a substrate such that itdoes not crack after cure or release from the substrate during dicing.Suitable polymers include PHILIPS type 40029 Resin or GAFGARD 233.Dashed lines 58 indicate the dicing lines for forming an individualintegrated element from the wafer. Preferably, the master element ismade of an elastomeric, polymeric material, particularlypolydimethylsiloxane (PDMS). As set forth in U.S. Pat. No. 6,027,595“Method of Making Optical Replicas by Stamping in Photoresist andReplicas formed thereby”, the PDMS master may also be used to embossphotoresist, which is hereby incorporated by reference in its entirety.

In the embodiment shown in FIG. 6a, the layer of embossable material 50may be provided on the master element 48. A layer of adhesion promoter54 is preferably provided on the substrate 52 and/or a layer of arelease agent is provided on the master element 48 in between the masterelement and the embossing material. The use of an adhesion promoterand/or release agent is of particular importance when the master and thesubstrate are of the same material or when the master naturally has ahigher affinity for adhesion to the embossable material.

The type of adhesion promoter used is a function of the photopolymer tobe used as the embossable material, the master material and thesubstrate material. A suitable adhesion promoter for use with a glasssubstrate is HMDS (hexamethyl disilizane). This adhesion promoterencourages better bonding of the embossable material onto the substrate52, which is especially critical when embossing on the wafer level,since the embossed wafer is to undergo dicing as discussed below.

The provision of the embossable layer 50 on the master 48 and of theadhesion promoting layer 54 on the substrate 52 advantageously providessmooth surfaces which are to be brought into contact for the embossing,making the elimination of air bubbles easier as noted below. Theprovision of the embossable layer on the master 48 also provides aconvenient mechanism for maintaining alignment of contacted, alignedwafers which have not been bonded, as discussed below.

If either the substrate or the master is made of plastic, it ispreferable to place the polymer on the other non-plastic component,since plastic absorbs strongly in the UV region used for activating thepolymer. Thus, if the UV radiation is required to pass through plastic,a higher intensity beam will be required for the desired effect, whichis clearly less efficient.

While the embossable material 50 is shown in FIG. 6a as being providedon the master 48, it is difficult to control the thickness of theembossable material 50 when applied to the master 48. Since it is oftendesirable for the embossable material 50 to be as thin a layer aspossible while still receiving the pattern, it can be advantageous toprovide the embossable material 50 on the substrate 52, e.g., byspinning on the photoresist or the epoxy, as shown in FIG. 6b. Thickerlayers of embossable material 50 result in much longer etching times,leading to increased expense, increased likelihood that the patternedmaterial will degrade due to the increased exposure to the etchingprocess, and increased inaccuracies due to deviations in etch rateacross the element. The thickness of the embossable material 50 providedon the substrate 52 can be accurately controlled in a conventionalmanner.

When placing the master on the substrate, the wafer cannot be broughtstraight down into contact. This is because air bubbles which adverselyaffect the embossed product would be present, with no way of removingthem.

Therefore, in bringing the master into contact with the substrate, themaster initially contacts just on one edge of the substrate and then isrotated to bring the wafer down into contact with the substrate. Thisinclined contact allows the air bubbles present in the embossablematerial to be pushed out of the side. Since the master is transparent,the air bubbles can be visually observed, as can the successfulelimination thereof As noted above, it is the presence of these airbubbles which make it advantageous for the surfaces to be brought intocontact be smooth, since the diffractive formed on the surface of themaster 48 could trap air bubbles even during such inclined contact.

When made of a flexible material, such as an elastomeric, polymericmaterial, the flexible master 48 may be bowed to allow a central portion48 b thereof to stamp the embossable material 50 first, and then releaseto allow outer portions 48 a, 48 c to stamp the embossable material 50as shown in FIG. 6b. Any air in the embossable material 50 is thusallowed to escape from the periphery as the master 48 is brought intofull contact with the embossable material.

The use of embossing on the wafer level is of particular interest whenfurther features are to be provided on the wafer using lithographicprocesses, i.e., material is selectively added to or removed from thewafer. Such further features may include anti-reflective coatings orother features, e.g., metalization pads for aligning the die diced fromthe substrate 52 in a system, on the embossed layer. Any such featuresmay also be lithographically provided on an opposite surface 56 of thesubstrate 52.

Typically an anti-reflective coating would be applied over the entiresurface, rather than selectively. However, when using both ananti-reflective coating and metal pads, the metal would not adhere aswell where the coating is present and having the coating covering themetal is unsatisfactory. Further, if the wafer is to be bonded toanother wafer, the bonding material would not adhere to the surface ofhaving such an anti-reflective coating, thereby requiring the selectivepositioning of the coating.

For achieving the alignment needed for performing lithographicprocessing in conjunction with the embossing, fiducial marks as shown inFIG. 3 may be provided on both the substrate 52 and the master 48. Whenperforming lithographic processing, the alignment tolerances requiredthereby make glass more attractive for the substrate than plastic. Glasshas a lower coefficient of thermal expansion and glass is flatter thanplastic, i.e., it bows and warps less than plastic. These features areespecially critical when forming elements on a wafer level.

The degree of the inclination needed for removing the air bubblesdepends on the size and depth of the features being replicated. Theinclination should be large enough so that the largest features are nottouching the other wafer across the entire wafer on initial contact.

Alternatively, if the replica wafer is flexible, the replica wafer maybe bowed to form a slightly convex surface. The master is then broughtdown in contact with the replica wafer in the center and then thereplica wafer is released to complete contact over the entire surface,thereby eliminating the air bubbles. Again, the amount of bow requiredis just enough such that the largest features are not touching the otherwafer across the entire wafer on initial contact.

When using the fiducial marks themselves to align the master element 48to the glass substrate 52 in accordance with the present invention, aconventional mask aligner may be used in a modified fashion. Typicallyin a mask aligner, a mask is brought into contact with a plate and thena vacuum seals the mask and plate into alignment. However, a vacuumcannot be created when a liquid, such as a polymer, embossable materialis on top of a wafer. Therefore, the above inclined contact is used.Once contact is established, the wafers are aligned to one another in aconventional fashion using the fiducial marks before being cured.

Further, the intensity required to cure the polymer is very high, e.g.,3-5 W/cm², and needs to be applied all at once for a short duration,e.g., less than 30 seconds. If enough energy and intensity are notapplied at this time, hardening of the polymer can never be achieved.This is due to the fact that the photoinitiators in the polymer may beconsumed by such incomplete exposure without full polymerization.

However, it is not easy to provide such a high intensity source with themask aligner. This is due both to the size and the temperature of thehigh energy light source required. The heat from the high energy sourcewill cause the mask aligner frame to warp as it is exposed to thermalvariations. While the mask aligner could be thermally compensated orcould be adapted to operate at high temperatures, the following solutionis more economical and provides satisfactory results.

In addition to the inclined contact needed for placing the master infull contact with the substrate in the mask aligner, once such fullcontact is achieved, rather than curing the entire surface, a deliverysystem, such as an optical fiber, supplies the radiation from a UVsource to the master-substrate in contact in the mask aligner. Thedelivery system only supplies UV radiation to individual spots on thepolymer.

The delivery system is small enough to fit in the mask aligner and doesnot dissipate sufficient heat to require redesign of the mask aligner.When using an optical fiber, these spots are approximately 2 mm.Alternatively, a UV laser which is small and well contained, i.e., doesnot impose significant thermal effects on the system, may be used.

The delivery system provides the radiation preferably to spots in theperiphery of the wafer in a symmetric fashion. For a 4 inch wafer, onlyabout 6-12 spots are needed. If additional spots are desired forincreased stability, a few spots could be placed toward the center ofthe wafer. These spots are preferably placed in the periphery and aminimal number of these spots is preferably used since an area where atack spot is located does not achieve as uniform polymerization as theareas which have not been subjected to the spot radiation.

These tack spots tack the master in place with the substrate. Theillumination used for curing the tack spots is only applied locally andthere are few enough of these tack spots such that the area receivingthe illumination is small enough to significantly affect the rest of theembossable material. Once alignment has been achieved and the mastertacked into place, the substrate-master pair is removed from the alignerand then cured under the high intensity UV source over the entiresurface for full polymerization. The tack spots prevent shifting of thealignment achieved in the mask aligner, while allowing thesubstrate-master pair to be removed from the mask aligner to thereby usethe high energy light source external to the mask aligner for curing thepolymer.

Alternatively, the fiducial marks may be used to form mechanicalalignment features on the perimeter of the surfaces to be contacted. Themechanical alignment features may provide alignment along any axis, andthere may be more than one such mechanical alignment feature. Forexample, the stand-offs in FIG. 4 are for aligning the wafers along they axis, while the metal pads provide alignment of the wafer pair toadditional elements along the x and z axes. The alignment features arepreferably formed by the embossing itself.

The embossing and the lithographic processing on the opposite surfacemay be performed in either order. If the embossing is performed first,it is advantageous to leave the master covering the embossed layer untilthe subsequent processing on the opposite surface is complete. Themaster will then act as a seal for the embossed structure, protectingthe polymer from solvents used during lithographic processing andkeeping the features accurate throughout heating during lithographicprocessing.

If the lithographic processing is performed first, then more precisealignment is required during embossing to provide sufficient alignmentto the photolithographic features than is required during normalembossing. Thus, embossing equipment is not set up to perform suchalignment. Then, the above alignment techniques are required duringembossing.

Once all desired processing has been completed, the wafer is diced toform the individual elements. Such dicing places mechanical stresses onthe embossed wafer. Therefore, full polymerization and sufficientadhesion of the embossed portion to the substrate is of particularimportance so that the embossed portion does not delaminate duringdicing. Therefore, care must be taken in selecting the particularpolymer, an adhesion promoter, and the substrate, and how these elementsinteract. Preferably, in order to avoid delamination of the embossedlayer during dicing, the adhesion of the polymer to the substrate shouldbe approximately 100 grams of shear strength on a finished die. If onlydicing a single wafer, the wafer could be flipped over and covered witha material which seals elements so that when the dicing slurry is used,it does not contact the elements. Such materials, which are easilyremoved after dicing is completed include Krylon and photoresist.Alternately, another wafer, with or without further optical elements,could be bonded to the wafer containing the stamped photoresist featuresin a manner described above such that a seal is formed between thewafers around each individual portion to be diced

For any of the above techniques, when formed in photoresist, the patternin the photoresist may be transferred to the substrate using knownetching techniques, or the photoresist itself may serve as the opticalelement. The sealing of the present invention is particularly importantwhen using optical element made of photoresist, since the dicing slurryadheres very well to the photoresist, making it very difficult to cleanthe photoresist element When both wafers to be bonded together as shownin FIGS. 1-4 have been embossed with a UV cured polymer, the typicalpreferred use of a UV epoxy for such bonding may no longer be thepreferred option. This is because the UV cured polymer will still highlyabsorb in the UV region, rendering the available UV light to cure theepoxy extremely low, i.e., in order to provide sufficient UV light tothe epoxy to be cured, the intensity of the UV light needed is veryhigh. Therefore, the use of thermally cured resin to bond such wafers issometimes preferred.

Alternatively, polymer on the portions not constituting the elementsthemselves may be removed, and then the UV epoxy could be employed inthese cleared areas which no longer contain the UV polymer to directlybond the glass substrate wafer having the UV polymer with another wafer.A preferably way to remove the polymer includes providing a pattern ofmetal on the master. This metal blocks light, thereby preventing curingof the polymer in the pattern. When a liquid polymer is used, thisuncured polymer may then be washed away. Other materials, such as the UVepoxy for wafer-to-wafer bonding or metal for active element attachmentor light blocking, may now be placed where the polymer has been removed.

In addition to the bonding of the two substrates shown in FIGS. 1-4d,the alignment marks may be used to produce optical elements on the otherside of the substrate itself, at shown in FIG. 7. The creation may alsooccur by any of the methods noted above for creating optical elements.The double sided element 70 in FIG. 7 has a diffractive element 72 on afirst surface 70 a thereof and a refractive element 74 on a secondsurface 70 b thereof, but any desired element may be provided thereon.Again, metal pads 76 may be provided through lithographic processing onthe hybrid element.

A further configuration of an integrated multiple optical elements isshown in FIG. 8, in which a diffractive element 82 is formed directly ona refractive element 84. The refractive element may be made by any ofthe above noted photolithographic techniques. In the specific exampleshown in FIG. 8, the refractive element is formed by placing a circularlayer of photoresist 86 on a surface of optical material using a mask.The photoresist is then partially flowed using controlled heat so thatthe photoresist assumes a partially spherical shape 87. Thereafter, thesurface is etched and a refractive element 84 having substantially thesame shape as the photoresist 87 is formed by the variable etch rate ofthe continually varying thickness of the photoresist 87. The microlens84 is then further processed to form the diffractive element 82 thereon.The diffractive element may be formed by lithographic processing orembossing.

The wafers being aligned and bonded or embossed may contain arrays ofthe same elements or may contain different elements. Further, whenalignment requirements permit, the wafers may be plastic rather thanglass. The integrated elements which are preferred to be manufactured onthe wafer level in accordance with the present invention are on theorder of 100 microns to as large as several millimeters, and requirealignment accuracies to ±1-2 microns, which can be achieved using thefiducial marks and/or alignment features of the present invention.

When the optical elements are provided on opposite surfaces of asubstrate, rather than bonded facing one another, tolerable alignmentaccuracies are ±10 microns. This is due to the fact that when light istransmitted through the thickness of the glass, slight amounts of tiltcan be corrected or incorporated.

As an alternative to the fiducial marks used for passive alignment, thefiducial marks may be used to create mechanical alignment features, suchas corresponding grooves joined by a sphere, metalization pads joined bya solder ball, and a bench with a corresponding recess. Only a few ofthese alignment features is needed to align an entire wafer.

All of the elements of the present invention are advantageously providedwith metalized pads for ease of incorporation, including alignment, intoa system, typically including active elements. The metalized pads mayefficiently be provided lithographically on the wafer level.

The invention being thus described, it would be obvious that the samemay be varied in many ways. Such variations are not regarded as adeparture from the spirit and scope of the invention, and suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for forming an integrated opticalsubsystem comprising: providing a bonding material surrounding each diein an array of first dies on a first wafer; aligning a plurality ofsecond dies with the first dies, each first die having a second diealigned therewith, at least one of said array of first dies and saidplurality of said second dies include a corresponding number oflithographically formed individual passive optical elements; treatingthe bonding material to thereby bond the aligned dies; and dicing thebonded dies, each diced, bonded pair of dies containing at least onepassive optical element, thereby forming an integrated opticalsubsystem.
 2. The method according to claim 1, wherein said providingincludes providing the bonding material over at least part of theoptical path of each die on the first wafer.
 3. The method according toclaim 1, wherein said providing includes providing the bonding materialover an entire surface of each die on the first wafer.
 4. The methodaccording to claim 1, wherein said providing includes sealing aperimeter of each die.
 5. The method according to claim 1, wherein thesecond dies are on a second wafer and said aligning includes aligningthe first and second wafers.
 6. The method according to claim 1, whereinthe second dies are separate from one another and said aligning includesaligning each second die with a corresponding first die.
 7. The methodaccording to claim 1, wherein one of said first and second dies is asemiconductor die.
 8. The method according to claim 7, wherein asemiconductor die contains a vertical cavity side emitting laser.
 9. Themethod according to claim 7, wherein a semiconductor die contains adetector.
 10. The method according to claim 7, further comprisingmounting discrete devices on one of said first and second dies.
 11. Themethod according to claim 10, wherein said mounting includes mounting atleast one of a mirror and a laser.
 12. The method according to claim 10,wherein said mounting includes mounting optics.
 13. The method accordingto claim 10, wherein said mounting includes mounting an optical fiber.14. The method according to claim 10, wherein one of said first andsecond dies is a polarizer.
 15. The method according to claim 1, whereinthe at least one passive optical element is formed of a materialdifferent from a material of its respective substrate.
 16. The methodaccording to claim 15, wherein the at least one passive optical elementis formed in photoresist.
 17. The method according to claim 1, whereinthe at least one passive optical element is formed in its respectivesubstrate.
 18. The method according to claim 1, further comprising,before said dicing, aligning a plurality of third dies with said firstand second dies.
 19. The method according to claim 18, wherein the firstdie is a wafer of collimating elements, the second die is a wafer offocusing elements, and the third die is a wafer serving as a spacerbetween the first and second dies.
 20. The method according to claim 1,wherein at least one of the first and second dies is made of silicon.